The ongoing integration and miniaturization of components for electronic circuitry has become a growing challenge to the limits of printed wiring board technology over the last twenty years. Printed circuit boards or printed wiring boards (PWB) as they are more commonly termed, play several key roles. First, the electrical components, such as specially packaged integrated circuits, resistors, etc., are mounted or carried on the surface of the flat usually sturdy card-like board. Thus, the PWB serves as a support for the components. Secondly, using chemically etched or plated conductor patterns on the surface of the board, the PWB forms the desired electrical interconnections between the components. In addition, the PWB can include a metal area serving as a heat sink.
Conductor patterns typically are formed by photoetching a copper foil clad epoxy fiberglass substrate. A photoresist layer is applied to the copper foil and patterned by exposure to ultraviolet (UV) light projected through a mask, often referred to as "artwork", e.g., to a positive art work image of the circuit pathways and contacts. Those areas exposed to the light are altered and are removed by treatment with a solvent for the resist, leaving areas of copper, e.g., in the desired conductor pattern, underneath the protective barrier of the remaining photoresist. The exposed copper is etched away and the remaining photoresist is then chemically removed to expose the resulting conductor pattern. Alternatively, the photoresist can be patterned to form channels for electroless plating of conductor patterns. There are, of course, many variations on this procedure, but all of them require photo-patterning of the resist layer.
Increased use of integrated circuits, and surface mount technology (SMT) has accelerated the densification of electronic circuitry. Surface mount devices (SMD) are applied directly to the surface of the PWB and soldered using vapor phase, infra-red (IR) or other mass soldering techniques. SMT is revolutionizing the electronic manufacturing industry by reducing assembly cost by about 50%, increasing component density by over 40% and enhancing reliability. The array of terminals on SMD's has a higher density or finer pitch then those on conventional components. As each terminal still has to be properly electrically connected to the respective conductor on the board, registration of SMD's requires high resolution for the PWB conductor lines. Indeed, SMD circuitry has become so dense that double-sided boards cannot accommodate all of the needed electrical connections. Thus, multilayer PWB's have become the focus of attention and several competing technologies are evolving. Those techniques which rely on stacks or layers of conductor patterns have interlayer registration requirements in additional to the exacting line width and spacing of a conductor pattern in a given layer. Manufacturing very fine lines on the order of 3 to 5 mils in registration over four or more layers deep is very difficult.
To take fullest possible advantage of the benefits offered by the emerging SMT, new fabrication processes must be developed in the manufacture of substrates and boards. In the past, one of the problem areas in fabrication of PWB's has been the generation and use of artwork masters for patterning the photoresist layers. Using photographic film or glass plates poses inherent difficulties in stability, registration, transport and storage.
In order to eliminate artwork masters the industry has fostered the development of UV laser plotters. These machines pattern the UV sensitive resist directly without artwork. Conductor patterns are designed using computer-assisted design (CAD) which digitizes the coordinates and dimensions of all of the paths and converts them to control signals for a UV laser x-y plotter. However UV laser plotters have a number of limitations, particularly when used for fine line, high density work. Principal among these is the fact that UV sensitive resists are relatively insensitive materials, requiring high levels of exposure energy. As a result, line edge resolution is limited. In order to achieve high plot speeds, these systems operate in a raster scan mode. Raster scanning produces considerable edge irregularities which are particularly apparent in plotting angled lines. Limitations in accuracy and minimum line width are characteristic of existing raster plotting systems. Another problem of current raster plotting systems is the short life expectancy of the laser source. A further problem with direct-from-CAD UV plotting of the photoprocessible layer is that such systems do not permit inspection before polymerization. If an error is made in the plot, the mistake is indelibly embedded in the UV sensitive layer. In the case of a resist, the board may be salvaged only by removing the entire resist layer and starting over after cleaning and baking the board free of moisture a second time. In the case of a UV plotted solder mask, a glitch in the pattern may result in the entire panel being discarded. Also, UV laser plotters are very expensive, and are relatively slow.
The foregoing discussion of the prior art is taken largely from Lake et al, U.S. Pat. No. 4,666,818 who propose a method for fabricating a printed circuit board utilizing two photo-reactive coatings. According to Lake et al a photo-processable ultraviolet sensitive layer is overlayed with a thin, unexposed, and undeveloped (silver halide) photographic film. A CAD system, containing within it the desired pattern layout for the interconnection lines, drives a white light X-Y plotter to expose the silver halide film on the board in the desired pattern, without effecting the underlying ultraviolet sensitive layer. The film is then developed and used as an in-situ mask for patterning the ultraviolet layer during an exposure of the board to ultraviolet light. The silver halide photographic film is not further affected by the exposure of the board to ultraviolet light. After the ultraviolet exposure, the silver halide photographic film is peeled off, to expose the resist, and the resist-coated board is processed further according to conventional methods.
Although capable of producing somewhat respectable interconnection line resolution and definition, the teachings of this patent result in serious drawbacks. First, even though Lake et al. teaches use of a white light x-y plotter and not a UV laser, all of the drawbacks associated with the use of a UV laser, e.g. speed, cost, etc., are presented by Lake et al. Moreover, the presence of the photographic film substrate introduces optical problems which results in reduced interconnection line definition and resolution due to white light diffusion through the film.